Question: On the first 4 exams of his geometry class, William got an average score of 84. What does he need on the next exam to have an overall average of 87?
Answer: Let his score on the next exam be $x$ The sum of all of his scores is then $4 \cdot 84 + x$ The same sum must also be equal to $5 \cdot 87$ Solve: $x = 5 \cdot 87 - 4 \cdot 84 = 99$.